Within the field of electrochemistry, there is a well known type of electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas an caustic soda, viz., sodium hydroxide, are produced by passing an electric current through a concentrated salt (brine) solution containing sodium chloride and water. A large portion of the chlorine and caustic soda for the chemical and plastic industries is produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane such as the commercially available NAFION manufactured by the E. I. duPont de Nemours and Co. Alternatively the separator can be a porous diaphragm, e.g., asbestos which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the caustic soda and so special precautionary measures and techniques have been employed in an attempt to reduce damage and deactivation of the active layer particles contained in the cathodes used in chlor-alkali cells.
Fairly recently attention has been directed in chlor-alkali cell technology to various forms of oxygen (air) cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed to operate chlor-alkali cells. Estimates indicate that there is a theoretical savings of about 25% of the total electrical energy required to operate chlor-alkali cells provided that the formation of molecular hydrogen gas at the cathode can be prevented. In other words about 25% of the electrical energy employed in a chlor-alkali cell is used to form hydrogen at the cathode. Hence the prevention of hydrogen formation at the cathode can lead to significant savings in the cost of electrical power. This is one of the major benefits of and purposes for oxygen (air) cathodes. However, such cathodes, being in contact with the electrolyte caustic soda, are subjected to the corrosive action thereof.
One known form of oxygen (air) cathode involves use of an active cathode layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed using precious metal catalyst materials, such as, silver, platinum, etc. The active carbon particles become wetted (flooded) by the caustic soda thereby significantly reducing their ability to eliminate the formation of hydrogen at the cathode and resulting in a loss of activity of the air cathode. Some attempts to overcome this difficulty involve incorporating hydrophobic materials, e.g., polytetrafluoroethylene (PTFE) in such active layers in particulate or fibrillated (greatly attenuated and elongated form) to impart hydrophobicity to the active carbon layer, per se. With the PTFE, however, comes the problem of reduced electrical conductivity in the cathode active layer in as much as PTFE, per se, is non-conductive when compared with the porous active carbon particles. Some oxygen (air) cathodes contain PTFE in both the active layer and in a backing sheet laminated thereto. The PTFE has been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the desired layer. Thus it can be seen that the development of corrosion-resistant oxygen (air) cathodes of improved durability for use in conjunction with chlor-alkali cells is an overall objective in the oxygen (air) cathode field.
U.S. Pat. No. 4,058,482 discloses a sheet material, principally comprised of a polymer such as PTFE and a pore-forming material wherein the sheet is formed of co-agglomerates of the polymer and the pore former. This patent teaches mixing polymer particles with postively charged particles of a pore former, e.g., zinc oxide, to form co-agglomerates thereof followed by mixing same with a catalyst suspension so as to form co-agglomerates of catalyst and polymer-pore-former agglomerates followed by pressing, drying, and sintering these co-agglomerates. Subsequent to this sintering, the pore former can be leached out of the electrodes.
U.S. Pat. No. 4,150,076 (a division of U.S. Pat. No. 4,058,482), is directed to the process for forming the sheet of U.S. Pat. No. 4,058,482, said process involving formation of polymer-pore-former co-agglomerates, distributing same as a layer on a suitable electrode support plate, for example a carbon paper, to form a fuel cell electrode by a process which includes pressing, drying, sintering, and leaching.
U.S. Pat. No. 4,170,540 to Lazarz et al discloses microporous membrane material suitable for electrolytic cell utilization and formed by blending particulate polytetrafluoroethylene, a dry pore-forming particulate material, and an organic lubricant. These three materials are milled and formed into a sheet which is rolled to the desired thickness, sintered, and subjected to leaching of the pore-forming material. According to the present invention, when forming the sheet by passing the fibrillated mixture of PTFE-particulate pore-forming agent through the rollers, special care is taken to avoid conditions which would cause the PTFE to sinter. The present invention is clearly distinguishable from U.S. Pat. No. 4,170,540 in respect of preparation of the backing sheet.
British Pat. No. 1,284,054 to Boden et al, is directed to forming an air-breathing electrode containing an electrolyte within an air-depolarized cell. This air-breathing electrode is made by hot pressing a fluoropolymer sheet containing a pore-forming agent on to a catalyst composition (containing silver) and a metallic grid member. According to page 3 of said British patent, the PTFE-pore-forming agent-paraffin wax containing sheet, is subjected to a solvent wash to remove the paraffin wax and then sintered in a sintering furnace at the appropriate temperatures for sintering the fluorocarbon polymer. After the PTFE-containing sheet is sintered and while it still contains the pore-forming particles, it is then ready for application to the catalyst composition of the air electrode for the hot pressing operation. Hot pressing involves the use of pressures ranging from about 5,000 to about 30,000 psi in conjunction with temperatures ranging from about 200.degree. F. to 400.degree. F.
The process of the present invention is readily distinguishable from British Pat. No. 1,284,054 in that the present invention avoids the use of a wax, avoids the trouble and expense of removing the wax with a solvent wash and does not use sintering thereby imparting greater porosity to the PTFE in fibrillated form in the finished electrode. Additionally the present invention avoids the repeated stripping-folding over-rolling again procedures required in all the examples of British Pat. No. 1,284,054. It will be observed that one of the backing layers which can be laminated according to the present invention surprisingly allows the formation of a porous, self-sustaining, coherent backing sheet or layer of PTFE using only a single pass through rollers.
U.S. Pat. No. 3,385,780 to I-Ming Feng discloses a thin, porous electrode consisting of a thin layer of a polytetrafluoroethylene pressed against a thin layer of polytetrafluoroethylene containing finely divided platinized carbon, the platinum being present in amounts of 1.2 to 0.1 mg/cm.sup.2 in the electrically conductive face of the thin electrode, viz., the side containing the platinized carbon, viz., the active layer. A thermally decomposable filler material can be used, or the filler can be a material capable of being leached out by either a strong base or an acid. U.S. Pat. No. 3,385,780 also mentions a single unit electrode involving finely divided carbon in mixture with PTFE.
In accordance with one embodiment of this invention in respect of the backing layer, partially fluorinated acetylene black carbon particles are incorporated with the PTFE in the backing layer thereby resulting in improved electrical conductivity in the backing layer combined with balanced hydrophobicity.
U.S. Pat. No. 4,135,995 to Cletus N. Welch is directed to a cathode having a hydrophilic portion formed of a solid intercalation compound of fluorine and carbon of the empirical formula CF.sub.x, where x ranges from about 0.25 to 1 and preferably ranges from about 0.25 to 0.7. The intercalation compounds of carbon and fluorine are referred to as hydrophilic, fluorinated graphites and graphite fluorides characterized by an infra-red spectrum showing an absorption band at 1220 cm.sup.-1. A layer of hydrophobic material, such as polyperfluoroethylene (polytetrafluoroethylene) can be utilized in a hyrophobic portion of the same layer or it can be utilized in the form of a different layer which can be associated with a current carrier layer. The Welch cathode may be utilized as an oxygen (air) cathode.
The present invention in respect of the backing layer is readily distinguishable from that of the Welch patents (when incorporating partially fluorinated acetylene carbon black particles) in several respects. First, the partially fluorinated compounds utilized in accordance with this invention have a hydrophobicity greater than that of the acetylene carbon black prior to partial fluorination. Secondly, the partially fluorinated compounds which can be utilized in accordance with one embodiment of this invention are acetylene carbon blacks of the formula CF.sub.x, wherein x ranges from about 0.1 to 0.18. Hence the extent of fluorination is markedly less in these partially fluorinated compounds as compared with those disclosed by said Welch patent. Thirdly it will be observed that the Welch intercalation compounds are fluorinated graphites or graphite fluorides. The partially fluorinated acetylene carbon black compounds which can be used in the laminates of this invention are partially fluorinated carbon-black, e.g. acetylene black, which acetylene blacks are produced by the explosive or thermal cracking of acetylene, or by corresponding electrical procedures. Such acetylene carbon blacks show significant differences when compared with graphitic blacks and active carbons due to their structure and history of production.
U.S. Pat. No. 3,838,064 to John W. Vogt et al is directed to a process for dust control involving mixing a finely divided fibrillatable polytetrafluoroethylene with a material which characteristically forms a dust to form a dry mixture followed by sufficient working to essentially avoid dusting. Very small concentrations of PTFE, e.g., from about 0.02 to about 3% by weight are employed to achieve the dust control. Corresponding U.S. Pat. No. 3,838,092 also to Vogt et al is directed to dustless compositions containing fibrous polytetrafluoroethylene in concentrations of about 0.02% to less than 1%, e.g., about 0.75% by weight of PTFE based on total solids.
The active layers whose use is contemplated to form the laminated three-layer electrodes in accordance with this invention are readily distinguishable from both the John W. Vogt et al patents (U.S. Pat. Nos. 3,838,064 and 3,838,092) and employ much higher concentrations of PTFE and for different purposes than are taught by said Vogt et al patents.
An article entitled "ON THE EFFECT OF VARIOUS ACTIVE CARBON CATALYSTS ON THE BEHAVIOR OF CARBON GAS-DIFFUSION AIR ELECTRODES: 1. ALKALINE SOLUTIONS" by I. Iliev et al appearing in the Journal of Power Sources, 1 (1976/1977) 35, 46, Elsevier Sequoia S.A., Lausanne-printed in the Netherlands, at pages 35 to 46 of said Journal there are described double layer fixed-zone, Teflon-bonded carbon electrodes having a gas supplying layer of carbon black "XC" wet proofed with 35% Teflon and an active layer consisting of a 30 mg/cm.sup.2 mixture of the same wet-proof material "XC-35" and active carbon "weight ratio of 1:2.5.". These electrodes were sintered at 350.degree. C. under a pressure of 200 kg/cm.sup.2 and employed as oxygen (air) cathodes in alkaline test environments.
The present invention is readily distinguishable from the oxygen (air) cathodes described by Iliev et al in that according to this invention, larger active carbon particles are "Teflonated" (discontinuously coated with much smaller PTFE particles) with subsequent fibrillation followed by heat treating and forming into a coherent, self-sustaining sheet without sintering. The active layers when incorporated into an electrode, result in an active layer having a desirable combination of tensile strength with resistance to blistering under high current densities in use. It will be observed the conditions employed in formation of the active layer are insufficient to effect sintering of the PTFE contained therein.
British Pat. No. 1,284,054 to Boden et al is directed to forming an air-breathing electrode containing an electrolyte within an air-depolarized cell. This air-breathing electrode is made by hot pressing a fluoropolymer sheet, containing a pore-forming agent, on to a catalyst composition (containing silver) and metallic grid member. According to page 3 of said British patent, the PTFE-pore-forming agent-paraffin wax containing wetproofing sheet is subjected to a solvent wash to remove the paraffin wax (lubricant and binder) and then sintered in a sintering furnace at the appropriate temperatures for sintering and while it still contains the pore-forming particles. It is then ready for application to the catalyst composition of the air electrode for the hot pressing operation. Hot pressing involves the use of pressures ranging from about 5,000 to about 30,000 psi in conjunction with temperatures ranging from 200.degree. F. to 400.degree. F. The process of the present invention is readily distinguishable from said Boden et al British Patent in that the present invention avoids the use of wax, avoids the trouble and expense of removing wax and does not employ sintering and high pressures. Moreover, Boden et al do not use a discrete active layer containing catalyst in their process.
The publication "Advances in Chemistry Series", copyright 1969, Robert F. Gould (Editor), American Chemical Society Publications, contains at pages 13 to 23 an article entitled "A Novel Air Electrode" by H. P. Landi et al. The electrode described contains 2 to 8 percent PTFE, is produced without sintering and is composed of graphitic carbon (ACCO Graphite) or metallized graphitic carbon particles blended with a PTFE latex and a thermoplastic molding compound to form an interconnected network which enmeshes the filter particles. This blend is molded into a flat sheet and the thermoplastic is then extracted. The present process employs non-graphitic active carbons, significantly higher concentrations of PTFE in the active layer while avoiding the use of thermoplastic molding compound and avoiding the necessity to remove same. Also, the active layer of this invention is formed by rolling a prefibrillated granular mix and no molding step is necessary.
No indication is given by Landi et al as to the stability and durability of their air electrode and no life testing or data is included in said article.
U.S. Pat. No. 3,368,950 discloses producing fuel cell electrodes by electrochemically depositing a uniform noble metal coating on a thin less noble metal body, for example, platinum on gold; platinum on silver; palladium on silver; gold on silver; rhodium on silver; gold on copper; silver on copper; nickel on iron or platinum on iron.
U.S. Pat. No. 3,352,719 is directed to a method for making silver-catalyzed fuel cell electrodes by plating a silver catalyst on a carbon or nickel substrate.
British Pat. No. 1,222,172 discloses use of an embedded conductive metal mesh or screen (35) within a formed electrode (30) containing a particulate (34) matrix of polytetrafluoroethylene polymer particles (21) in which there are located dispersed electrically conductive catalyst particles (24) which can be silver-coated nickel and silver-coated carbon particles, viz., two different types of silver-coated particles in the PTFE particulate matrix in an attempt to overcome an increase in resistance as silver is consumed in the gas diffusion fuel cells to which said British patent is directed.
U.S. Pat. No. 3,539,469 is directed to the use of silver-coated nickel particles (powder) in a fuel cell catalyst to economize on the use of silver. This patent states that silver, as an oxygen activation catalyst, has been known and heretofor used.
None of the references mentioned herein disclose an asymmetric woven wire mesh current distributor which can be used in accordance with this invention.
The laminates of this invention contain active layers having from about 60 to about 85 wt.% active carbon, the remainder being unsintered fibrillated polytetrafluoroethylene in intimate admixture with said active carbon. These active layers, per se, are described and claimed in U.S. Pat. No. 4,379,772 entitled "Electrode Active Layer". The disclosure of this patent is incorporated herein by reference. While the hydrophobic backing layers of any one of U.S. patent application Ser. Nos. 202,582 U.S. Pat. No. 4,382,904; and 202,583 U.S. Pat. No. 4,339,325; and 202,575 and the woven asymmetric wire mesh of U.S. patent application Ser. No. 202,574 U.S. Pat. No. 4,354,917 can be used as the backing layer and current distributor, respectively, in the laminates of this invention; the present laminates can incorporate any backing layer and any current distributor, respectively, including those of the prior art disclosed herein. Of course, then such laminates will not possess the specific desirable characteristics obtainable in the specific laminates formed and referred to herein. Nevertheless the present invention in its broadest aspects embraces the active layer of U.S. Pat. No. 4,379,772 with any wetproofing (backing) layer and any current distributor.